The present disclosure relates to conductive connections such as solder connections. Such connections are used, for example, in packaging of semiconductor integrated circuits, and in particular in attaching an integrated circuit or a packaging substrate to another integrated circuit or substrate. Examples of packaging substrates include wiring substrates (e.g. printed circuit boards) and interposers.
Solder is widely used in such connections because, for example, a solder connection can be easily undone (by melting the solder) to repair or replace a defective component without replacing the entire package.
FIGS. 1A and 1B illustrate solder attachment of an integrated circuit 110A to a structure 110B. Structure 110B may be another integrated circuit, or a printed circuit board (PCB), or some other packaging substrate, or can be an integrated circuit package. Circuit 110A has electrical contacts (“contact pads”) 120A which must be soldered directly to contact pads 120B of structure 110B. Such direct solder attachment is called “flip-chip” attachment. (Alternatively, contact pads can be interconnected by discrete wires, but flip-chip attachments are preferred because they reduce the assembly size and they lower the electrical resistance, parasitic capacitance, and parasitic inductance.)
As shown in FIG. 1A, solder balls 140 are formed on pads 120A. (The solder balls may alternatively be formed on pads 120B or on both pads 120A and 120B.) Then the structures 110A and 120B are brought together so that the solder 140 physically contacts the contact pads 120A and 120B. This assembly is placed into an oven to melt the solder. The solder then cools and re-solidifies to attach the pads 120A to the corresponding pads 120B.
During manufacture and subsequent circuit operation, solder connections 140 can be pulled sideways by various forces. A common source of such forces is thermal expansion: structures 110A, 110B may expand or contract due to heating or cooling, and the structure 110A may expand or contract by a different amount than the structure 110B if the two structures have different coefficients of thermal expansion (CTE). Solder connections 140 may crack or break, impairing or destroying the electrical functionality. Reliability of solder connections is thus an important goal in designing a manufacturing process.
Solder connections can be made more reliable by increasing their height H. However, increasing the height also increases the solder ball's width W because the solder tends to become spherical when melted. As a result, adjacent pads 120A must be spaced farther apart so as not to be shorted to each other. The minimal distance between pads 120B is also increased. Of note, the distance between the centers of adjacent pads 120 cannot be smaller than the distance between the centers of the adjacent solder balls, i.e. the solder ball pitch; the pitch is therefore greater than or equal to W. The increased pitch undesirably increases the assembly size and may degrade the electrical functionality (for example, by requiring longer interconnect lines (not shown) and/or by making the circuit slower and/or more power-hungry).
Thus, it is desirable to increase the height H without the corresponding increase in W, i.e. to increase the ratio H/W (this ratio is about equal to the height-to-pitch ratio because the solder ball width W is about equal to the minimal solder ball pitch). In order to increase this ratio, a solder connection can be made as a stack of solder balls. FIG. 2 illustrates a stack 210 of four solder balls 140.1, 140.2, 140.3, 140.4 on a contact pad 120 of a structure 110. The stack can be produced by a solder jetting apparatus 220 of type SB2-Jet available from Pac Tech (Packaging Technologies USA, Inc.). See e.g. E. Zakel et al., “High Speed Laser Solder Jetting Technology for Optoelectronics and MEMS Packaging”, ICEP 2002, incorporated herein by reference. In this apparatus, melted or at least softened solder balls 140 are supplied one after another through a nozzle 240. Gas pressure is used to propel and place each solder ball on top of contact pad 120 or of another, previously placed solder ball. As each solder ball is being placed, the solder ball is melted by a laser source 250 and is then cooled to adhere to the contact pad 120 or the underlying solder ball. Solder stack 210 does indeed have a higher H/W ratio than a single solder ball, but the H/W increase is limited because the solder balls 140 are squashed (flattened) in the jetting process. The flattening acts to increase W and decrease H. For example, if each solder ball 140 has the same volume as a sphere of radius 250 then the stack of two solder balls 140.1, 140.2 may have the height H of only 400 μm or less, not 500 μm. The solder ball width W is higher than 250 μm, so the height-to-pitch ratio is smaller than 400/250=1.6 for the two-ball stack, or smaller than 0.8 per ball.
After solder stack formation, the structure 110 is attached to another structure (as in FIG. 1B). In this structure-to-structure attachment, the solder stack is reflowed (melted), and the ratio H/W may further decrease during reflow. For example, a 450 μm diameter solder ball may become only 360 μm high, requiring a pad-to-pad pitch of about 600 μm (unless the solder is laterally restrained by a mask, not shown). The entire solder stack may even collapse. To counteract this phenomenon, one of the balls 140 can be made of a high-melting-temperature material having a higher melting temperature than the other balls so as not to melt during the structure-to-structure attachment. Such a technique is described in U.S. Pat. No. 6,455,785 issued Sep. 24, 2002 to Sakurai et al. In that technique, the bottom balls 140.1 are made of gold, and the overlying balls of indium (indium has a lower melting temperature than gold). Each ball is formed by heating a tip of a metal wire (gold or indium wire, heated by a high voltage discharge or a gas flame): the molten tip forms a ball which is placed on the structure, and the wire is then cut off. A wire residue remains as a small protrusion on top of the ball, and these protrusions are flattened by a plate pressing on the balls. Then, in the structure-to-structure attachment, only the indium balls are reflowed, the gold ball remains solid to prevent collapse.
FIGS. 3A, 3B illustrates another bumping technique from a PhD Dissertation by Xingshen Liu entitled “Processing and Reliability Assessment of Solder Joint Interconnection for Power Chips”, PhD Dissertation, 2001 (URN etd-04082001-204805), Virginia Tech Digital Library, Chapter II, incorporated herein by reference. First, solder balls 140.1 are formed on contacts 120A from solder paste. Then higher-melting-temperature balls 140.2 are placed on balls 140.1, and solder 140.1 is reflowed. Balls 140.2 are never melted. Solder balls 140.3 are formed on contact pads 120B of another structure 110B from solder paste. Solder 140.3 has lower melting temperature than 140.1. Then structure 110A is placed on structure 110B, and the solder 140.3 is reflowed for attachment to solder 140.2. Solders 140.1 and 140.2 are not melted in this process.
Further improvements are desirable in forming conductive connections.